This invention relates generally to optical correlators, and, more particularly to a prismatic anamorphic system for use therewith.
Most topographic maps are made from aerial sterophotographs with the aid of photogrammetric measuring instruments capable of stereoperception. In recent years, the increasing demand for topographic maps has prompted the development of automatic stereoperception equipment which functions by accurately matching corresponding images in stereophotographs. All such equipment incorporates correlation-measurement techniques, which are ideally suited to the task.
The most successful automation to date has been achieved using flying-spot scanners in conjunction with electronic correlators. However, the general availability of reliable laser light sources for coherent optical processing now makes it practical to use optical correlation techniques, with a view to increasing speed and accuracy.
Two obvious advantages associated with optical correlation techniques are (1) an almost instantaneous processing speed and (2) a potentially high signal-to-noise performance, the latter being made possible by the greater intensity of the laser light as compared with the cathode-ray-tube (CRT) spot in the flying-spot scanner. For these advantages to be realized, however, an optical correlator is required that is capable of extracting certain measurements from stereoimages and is at the same time compatible with optical systems that compensate for relative image distortion.
In general, there are two types of optical photographic-image correlators, the two-image or image/image correlator and the image/matched-filter correlator. Both operate on well-documented principles. In the two-image correlator, as the name implies, two images are correlated directly. In the image/matched-filter correlator, correlation is between an image and a matched filter -- a holographic recording of the Fourier transform of the second image. Of the two, only the two-image correlator is compatible with present stereophotogrammetric instruments, which use two-photograph images as inputs.
To accurately match images in stereophotographs, the correlation technique used must be capable of extracting three types of measurement: alignment error, distortion error, and correlation quality. It must also incorporate means of compensating for first-order distortion and terrain roughness. Alignment-error signals are required for the automatic positioning of corresponding stereoimages in the correlator aperture and so are fundamental to the entire automatic image-matching process. Distortion-error and correlation-quality signals are less important, but they do contribute significantly to the accuracy and effectiveness with which image matching is accomplished.
The nature of the sterophotographic process is such that it produces relative image distortion. Corresponding stereoimages are exactly alike only if the terrain is flat and if perfectly vertical frame photographs are used. Relative camera tilt and terrain-elevation changes result in significant differences in appearance between the two images. When such distorted images are superimposed in the correlator, image registration is poor and correlation measurements and correlation-derived alignment-error signals are inaccurate. A high degree of matching accuracy can be maintained only if the images can be made to appear nearly identical to the correlator. An accurate automatic image-matching system must therefore incorporate techniques both for measuring relative image distortion and for compensating or correcting for it.
The basic configuration of the two-image optical correlator system incorporates a laser light source, a phototube detector, three transform lenses, and two spatial filters. The laser provides an intense beam of collimated coherent light that acts as the signal carrier throughout the optical system. It is directed down the optical axis toward the phototube and successively passes through each element of the optical train.
As the beam emerges from the first diapositive, it picks up all the pictorial information stored in the image. As it proceeds toward the transform lens, it fans out (a diffraction effect produced by the detail in the diapositive) carrying all the pertinent image information. At most, only half of the transmitted light can be diffracted. The transform lens distributes the diffracted light in a manner analogous to the Fourier frequency decomposition of signals. For the correlator to work effectively, the average background of the first image must be removed. This is accomplished by placing an opaque circular light block on the optical axis of the frequency plane, where this frequency component of the image lies. The process is called spatial filtering, and the specific filter used here is commonly referred to as a DC block because it removes the direct component from the image.
The amplitude distribution of the light emerging from the second stereo diapositive is the product of the detail in the first image and the unfiltered image in the second diapositive. The output lens takes a second successive Fourier transform of this product signal and displays it in its front focal plane. The correlation signal appears directly on the optical axis in the second frequency plane, this term being separated from the other light by a pinhole field stop. A phototube directly in line with the pinhole measures the intensity of the transmitted light. What is actually measured is the square of the absolute value of the correlation function, since the latter is expressed in terms of light amplitude rather than intensity; for purposes of this application, however, there is no significant difference between the characteristics of the correlation function and those of its squared value.
A major problem with the optical correlator is accommodating differential scale resulting from camera geometry and/or terrain slope. An especially common problem is one where sloping terrain, as a result of camera perspective displays a scale change in one direction and not the other. Heretofore corrective techniques were extremely cumbersome insofar as they involved the axial translation of three components or utilized zoom lenses, either spherical or cylindrical, which required an additional scale change since operation around unity magnification is precluded. Furthermore, as the focal length of the zoom is changed, the zoom lens must be translated axially. There clearly exists a need for a corrective system which permits operation about unity magnification, introduces no image rotation, and, with proper design requires no axial translation as the magnification is varied.